With progress in satellite telecommunications, continuous growth is observed in satellite capacity in terms of the number of communications channels that can transmit in this way via the satellite, and also in terms of the throughput of each channel. In order to maximize the efficiency of a satellite in terms of service given for money invested, attempts are made to extend satellite lifetime as much as possible while simultaneously increasing both the quantity and the quality of the service that can be provided.
To this end, it is necessary to minimize the use of any resource that is necessary for implementing and operating the space segment of a satellite telecommunications system, and in particular: the mass and the size of the vehicle and its on-board equipment; and on-board electrical power consumption; and it has already become necessary to conserve radio spectrum which is a limiting resource of which ever-increasing use is being made.
Several of these criteria are facilitated by using a plurality of narrow beams for down links, each beam being formed to optimize its "footprint" on the ground, so as to waste a minimum amount of energy radiated outside the region of the earth which is to be served by the beam. The beams are formed by high-gain antennas on the satellite, each beam having the main lobe of its radiating antenna pointing towards its service region.
Thus, a major portion of state of the art telecommunications satellites use an increasing number of narrower and narrower beams for down links. Depending on the type of antenna used for forming these beams, their positions (ground coverage) may be fixed or variable (steerable). In all cases, this progress gives rise to a problem that is at the core of the present invention. The satellite needs to pick up a signal coming from a given location, and to retransmit it to another location on a down link. The problem which then arises is a switching problem whereby the signal from a given receive antenna is conveyed through the electronics of the payload to a particular transmit antenna. The problem becomes more complex with growth in the number of channels or signals to be processed simultaneously, multiplied by the number of different destinations for said signals.
By analogy with the past of terrestrial telecommunications by radio beam, the term "channel" is used to mean a band of frequencies about a central frequency. Also for reasons of past history, this term as used in space refers to the authorized subdivision of the transmission spectrum into channels that are 36 MHz or 72 MHz wide, that being the conventional bandwidth of transponders for applications in telephony or in television. In contrast, for switching purposes, the concept of a channel may be generalized to that of a path followed by a signal between a source and a destination. For satellites that are presently in orbit, this generalization of the concept "channel" coincides with the above conventional definition.
In the prior art, satellite payload systems are known that enable channel switching to be performed on board the satellite, between an input port and an output port. The need for this switching capacity is fundamental, e.g. to ensure redundancy for key components that may fail. For example, in the event of a power amplifier breaking down, it is necessary to be able to switch signals to another, backup amplifier so as to be able to continue to provide the link.
Conventionally, such switching is performed by electromagnetic switches on waveguide transmission lines for channels having a bandwidth of 36 MHz or 72 MHz. In such systems, it is important to minimize the number of switches and waveguide lines because of the weight and the bulk of such components. As a result, this constraint constitutes a major constraint in the prior art with respect to designing payload architecture. Thus, transmission systems have been designed to maximize the traffic that can be conveyed by an architecture of this type in channels of 36 MHz or 72 MHz.
In particular, in order to be enable to convey the calls of an increasing number of customers through a payload whose number of channels must remain the same throughout the life of the satellite, signal encoding systems such as time division multiple access (TDMA) and frequency division multiple access (FDMA) have been devised. Signals encoded in this manner can convey the calls of a very large number of customers simultaneously, providing the customers use the same waveguide switch configurations as exist on board the satellite. In general, the configuration represents a path between a receive antenna and a transmit antenna on board the satellite, and by extension a path between an up link beam and a different down link, thus between an originating geographical location and a different destination geographical location on the terrestrial globe.
This kind of system clearly lacks flexibility as soon as numerous users that are geographically dispersed desire simultaneously to communicate with geographical locations that are covered by different beams. In addition, different users have different requirements concerning the bandwidth necessary for their calls, with this depending on the required data rate. An object of the present invention is to provide communications apparatus that enables a larger number of users to be served simultaneously, even in the event of very wide geographical dispersion between the originating locations of the signals and the destination locations of said signals, and which thus makes it possible to serve with maximum efficiency the variable-bandwidth requests of the various users.